The petroleum industry began in 1859 in Pennsylvania when the Drake well was drilled purposely to find liquid petroleum. The petroleum industry grew very slowly. It was not until 1901, with the discovery of the Spindletop field in Texas, that the world was convinced there was more petroleum than could ever be consumed. The birth of the automobile industry created a growing demand for petroleum products and an accelerated growth in the petroleum industry.
As the world had more petroleum than it could consume, the oil reservoirs found prior to 1940 were produced utilizing only the natural energies of the petroleum accumulation. In the late 30's waterflooding and gas injection were initiated in some petroleum reservoirs that had been depleted of their natural energies. These reservoirs were usually at pressures of less than 100 psi and were waterflooded at pressures of 400 to 800 psi.
After World War II the petroleum industry began utilizing more engineering in the development of petroleum reservoirs. This resulted in initiation of pressure maintenance by water injection and gas injection much earlier in the productive life of the reservoir. These injection projects were carried out at pressures usually in excess of 500 psi and often at pressures as high as 4000 psi. Oil remaining in these reservoirs contains considerable gas in solution.
In the late '50's it was already evident that finding new petroleum was becoming more difficult and more expensive. The world demand for petroleum had grown at an unpredicted rate. It became evident that the world would soon be short of petroleum. As a result, new processes were initiated in an attempt to recover more of the petroleum that had already been found. These new processes, fire-flooding, steam stimulation, steam flooding and misciable flooding, were initiated in an effort to increase the recovery from existing reservoirs. Most of these techniques were very expensive and in most cases did not achieve the desired degree of success. At present, the only process still being utilized is steam stimulation and steam flooding, in reservoirs with low gravity and viscosity oil.
In the mid '60's the industry began to investigate possibilities of tertiary recovery in reservoirs that had been water flooded in the mid '40's. Tertiary projects utilizing caustics, surfactants, special emulsions and polymers were initiated on an experimental basis. At the present time the Federal government, through ERDA, and the petroleum industry are continuing to investigate the tertiary recovery processes. None of these techniques have been classified an economic success with a great future potential of recovering large quantities of additional petroleum.
In the late 1960's the first large scale commercial mining of an oil sand was initiated in Alberta, Canada. This project is strip mining a "tar" sand and processing the sand to recover the hydrocarbon. The project was uneconomic at world oil prices prior to 1973. The project became economical after petroleum prices were increased. As of this writing other projects scheduled in the area have still not been placed on a production status because of economics.
Oil reservoirs which may be candidates for a mining recovery process can be classified into two general categories, depleted or virgin. The depleted reservoirs were estimated to contain some 300 billion barrels of unrecovered oil in a 1976 study by the National Petroleum Council and are estimated to comprise between 60 and 80 percent of the oil originally present in these fields. The depleted reservoirs can be classified into two general categories; those which have undergone some type of secondary recovery process and those which have only been primarily depleted. The former category would probably be classified as containing the greatest amount of remaining petroleum because they represent the greatest number of petroleum reservoirs. A considerable number of primary depleted reservoirs exist in the viscous crude oil category. Some of these reservoirs have undergone steam stimulation but not a secondary recovery process. The volume of oil in this category is probably in excess of 150 billion barrels.
The depleted systems which have undergone secondary recovery processes are the ones flooded in the late '30's and early '40's. These were at relatively shallow depths and which were at very low pressures. Some of these reservoirs were placed on a vacuum during World War II and hence would contain very small amounts of gas in solution in the oil. The low gas in solution results in very small amounts of any gas in the reservoir to be utilized as a displacing fluid.
The other type of reservoirs which have undergone secondary recovery will have been flooded at pressures in excess of 50 psi and will have considerable gas in solution and possibly free gas to assist in the removal of fluids from the formation.
These depleted reservoirs will probably contain oil having the following properties.
Viscosity--1 to 10 centiposes PA1 Gravity--greater than 25.degree. API PA1 Gas in Solution--between 10 and 800 std. cu. ft. per reservoir barrel PA1 Oil Saturation--between 10 and 40 percent of the pore space PA1 Porosity--between 15 and 30 percent
Whether these reservoirs will be feasible for a mining process will be a function of the product of the formation thickness, formation porosity and residual oil saturation.
Oil in Place=Area.times.Thickness.times.Porosity.times.Oil Saturation. Factors of depth and mineability of formations above or below the oil zone itself will be major factors in the economics or feasibility of any such process.
The reservoirs which have undergone primary depletion only, with or without steam stimulation, will normally contain a much higher percentage of inplace oil at the time any mining process may be initiated.
These reservoirs will normally contain a more viscous oil, be at relative low pressures, and at relatively shallow depths. These reservoirs are represented by the higher viscosity oil reservoirs in California, Venezuela, and in Canada. They are presently being produced but with great difficulty. There is probably over 500 billion barrels of oil in this class of reservoir in the world.
Virgin reservoirs which might be susceptible to underground mining are represented by the known tar sands and the very viscous or high pour point oil deposits throughout the world. It is known that very extensive reserves of petroleum exist in these type of deposits in the United States, Canada and South America. The estimated reserve is 1,000 billion barrels. These virgin deposits are susceptible to both strip mining and to some types of underground recovery.
The "tar" sands which are being produced in Canada by means of strip mining have been drilled and tested by conventional petroleum recovery mechanisms with very little success. The petroleum content of these "tar" sands change into a very viscous oil with depth. They have not been treated with combinations of known recovery technology in order to make them productive. They have been overlooked as a potential source of petroleum production primarily because of the quality of the petroleum and their location. In many cases these reservoirs represent a petroleum deposit which is directly mineable by surface methods. As the depth of these deposits increase, the contained petroleum is very highly viscous and has gas in solution so that removal of the petroleum containing formation is not possible because of gas release to the atmosphere in the pit and the resultant ventilation and fire-explosion hazard.
It is believed that using available mining technology it is possible to develop mine working preferably beneath the oil-water contact of certain oil fields in selected areas. By being below the oil-water contact, it should be possible to hold the greatest hazard to mining, the inflow of any of the gasses normally associated with oil production, to an acceptable minimum.
The concept of applying mining techniques to the production of oil from oil bearing sands, both virgin and depleted, is both old and well known as described in a United States Bureau of Mines Bulletin No. 351 entitled "Mining Petroleum by Underground Methods" by George S. Rice published by the U.S. Government Printing Office in Washington, D.C. in 1932. Methods for the underground mining of oil were further investigated and reported in a bulletin entitled "Mining For Petroleum: Feasibility Study" prepared for the U.S. Department of the Interior-Bureau of Mines under contract No. J0275002 July, 1978. These prior art publications while informative, do not include sufficient specific details of how to go about overcoming the many practical problems encountered in the underground mining of petroleum. To overcome this deficiency the present invention was devised.